An all-optical measurement technique called Impulsive Stimulated Thermal Scattering (ISTS) measures a variety of different material properties, such as film thickness. In ISTS, two or more excitation laser beams from an excitation laser overlap in time and space on a surface of a sample to form a spatially varying optical interference pattern. The excitation laser beam consists of a series of short (e.g., a few hundred picoseconds) optical pulses having a wavelength within the absorption range of the sample. The excitation pattern features alternating "light" (i.e. constructive interference) and "dark" (i.e. destructive interference) elliptical regions with a spacing that depends on the wavelength of the laser beams and the angle between them. The light regions of the pattern heat the sample, causing it to thermally expand. This launches coherent, counter-propagating acoustic waves whose wavelength and direction match the pattern.
For opaque films (e.g., metal films), the acoustic waves generate a time-dependent "ripple" pattern on the film's surface that oscillates at one or more acoustic frequencies (typically a few hundred megahertz). The acoustic frequency depends on film properties such as thickness, density, and elastic moduli. A probe beam then diffracts off the ripple to form a series of signal beams, each representing at least one distinct diffraction order (e.g., the +1, -1, +2, or -2 orders). The signal beams oscillate in intensity at the acoustic frequency or a multiple thereof, or at sums or differences of acoustic frequencies if several are present. One or more of the signal beams is detected and monitored to measure the properties of the sample.
Use of ISTS to measure film thickness and a variety of other properties is described, for example, in pending and issued U.S. Pat. No. 5,633,711 (entitled MEASUREMENT OF MATERIAL PROPERTIES WITH OPTICALLY INDICED PHONONS); U.S. Pat. No. 5,546,811 (entitled OPTICAL MEASUREMENT OF STRESS IN THIN FILM SAMPLES); and U.S. Ser. No. 08/783,046 (entitled METHOD AND DEVICE FOR MEASURING FILM THICKNESS, filed Jul. 15, 1996), the contents of which are incorporated herein by reference.
ISTS-measured film thickness can be used as a quality-control parameter during and/or after the manufacturing of microelectronic devices. In these devices, thin films of metals and metal alloys are deposited on silicon wafers and used as electrical conductors, adhesion-promoting layers, and diffusion barriers. For example, metal films of copper, tungsten, and aluminum are used as electrical conductors and interconnects; titanium and tantalum as adhesion-promoting layers; and titanium:nitride and tantalum:nitride as diffusion barriers. Thickness variations in the metal films can modify their electrical and mechanical properties, thereby affecting the performance of the devices in which they are used. To effectively monitor metal films in a fabrication process, the ISTS film-thickness measurement must therefore be highly repeatable, precise, and accurate.